Mark, don't know how you feel about singing/playing with mp3 files, but I believe sites such as Karaoke Version and others have male/female duets with one of the voices dropped out. If you're just using it to add some variety that might work.
Tony...yep, that's just what I've been thinking (for a while now also). Since I know I'm NOT going to end up with a live human being, I'll get some of those "to add some variety."
I have a few now and they really do the job. I want to sing the duets from the great musicals: West Side Story, Sound of Music, Showboat, etc and I downloaded the Quartet from Rigoletto.
That is something I will DEFINITELY do...get a few more Music Minus Male(but with female) MP3's! Not just for the audience but for my own enjoyment.